The AWI 1-2 Core Research Program: Part I, Geologic Overview of the Awibengkok Geothermal Field, Indonesia

Abstract

The Awibengkok geothermal system is hosted by a thick volcanic section consisting primarily of andesitic rocks, and floored by marine sedimentary rocks of Miocene age. The volcanic sequence is capped by Quaternary rhyolitic domes and related tuffs that form a NNE-trend across the eastern part of the field. A complex network of N to NNE-trending extensional faults linked with ENE- and NW-trending accommodation faults provides high permeability. Phreatic explosion breccias exposed at the surface date back at least 8,400 years. Shallow argillic alteration at Awibengkok gives way with increasing depth and temperature to an argillicphyllic zone (smectite and illite/smectite) to a propylitic zone (epidote, chlorite, and illite), which accounts for the bulk of the system°Øs production. Awibengkok is a high-temperature (220-315oC), liquiddominated reservoir, averaging about 1.3% total dissolved solids (TDS) and low to moderate gas content. The principal deep upflow zone (°´315oC) is in the western part of the field. Ascending fluids are locally confined by low permeability, clay-rich ash deposits on the west, but move up along one or more N- or NNE-trending structures that breach these barriers in the central and east-central parts of the field. Fluids in the central part of the reservoir are very uniform in composition and temperature, representing the mixing of upflow and convective reflux. Fluids ascend and flow laterally to the shallow top of reservoir in the eastern area near Awi 1-2 (250 to 270oC). This sector of the geothermal system has had a complex history related to episodic volcanic eruptions, faulting, and hydrothermal venting.